CONTROL CIRCUIT AND METHOD FOR A FLYBACK POWER CONVERTER
A flyback power converter includes a power switch connected to a primary side of a transformer, and a sensing signal is provided for a control circuit to switch the power switch so as for the transformer to convert an input voltage into an output voltage. The sensing signal is a function of the input voltage, and the control circuit extracts a variation of the sensing signal during a preset time period. The variation of the sensing signal is used to prevent the output ripple and the green mode entry point of the flyback power converter from varying with the input voltage.
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The present invention is related generally to a flyback power converter and, more particularly, to a control circuit and method for a flyback power converter.
BACKGROUND OF THE INVENTION
I1min=(Burst_level/Rcs)+(Vin/Lm)×Tp, [Eq-1]
where Lm is magnetizing inductance of the primary side of the transformer 14. After the power converter 10 enters the burst mode, the average frequency in each burst cycle will fall within an audible noise range of 100 Hz-20 kHz. Therefore, the higher the current I1 is, the louder the audible noise will be. Moreover, the feedback signal Vcomp varies with the peak value of the current I1.
Minimum on-time control can also reduce the switching frequency of the power switch 18 at no load or light load. However, given a constant minimum on-time at light load, the output voltage Vo will have significant ripple when the input voltage Vin is high, and when the input voltage Vin is low, the energy delivery in each cycle the power switch 18 is switched will become inefficient. As a result, it is impossible to significantly reduce the switching frequency.
Therefore, it is desired a control circuit and method to maintain the green mode entry point and modulate the minimum on-time depending on the variation of the input voltage.
SUMMARY OF THE INVENTIONAn object of the present invention is to provide a control circuit and method for a flyback power converter.
A flyback power converter includes a power switch connected to a primary side of a transformer, and a sensing signal and a first feedback signal are provided for a control circuit which generates a control signal to switch the power switch so as for the transformer to convert an input voltage into an output voltage. The sensing signal is a function of the input voltage, and the first feedback signal is a function of the output voltage. The control circuit comprises a sampling and holding circuit to extract a variation of the sensing signal during a preset time period, a compensation circuit to compensate the first feedback signal with the variation of the sensing signal to generate a second feedback signal varying with the input voltage, a pulse width modulation circuit to generate the control signal according to a clock, the sensing signal and the second feedback signal, a power-saving circuit to control the flyback power converter to enter a green mode according to the first feedback signal and a preset voltage, and a minimum on-time modulation circuit to determine a minimum on-time of the power switch according to the variation of the sensing signal.
These and other objects, features and advantages of the present invention will become apparent to those skilled in the art upon consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings, in which:
Referring to
ΔVcs=(Vin/Lm)×Δton×Rcs, [Eq-2]
which shows the variation ΔVcs of the sensing signal Vcs is proportional to the input voltage Vin. Therefore, by multiplying the variation ΔVcs of the sensing signal Vcs with a proper coefficient
K=Tp/Δton, [Eq-3]
the impact of signal propagation delay in the equation Eq-1, i.e., (Vin/Lm)×Tp, can be eliminated so that the current I1 on the primary side of the transformer 14 has substantially the same peak value regardless whether the input voltage Vin is high or low, and in consequence audible noise is reduced.
At light load, the flyback power converter 10 operates in a discontinuous conduction mode (DCM). In order for a same output ripple regardless of the input voltage Vin being high or low, it is necessary to control the peak value of the current I1 on the primary side of the transformer 14 at a constant value.
Tmin={[Vinmax×(Tmin_max+Tp)]/Vin}−Tp. [Eq-4]
The relationship between the input voltage Vin and the minimum on-time Tmin following the equation Eq-4 is shown by curve 92 in
Tmin=[CX(V2−ΔVcs×4)]/(Gm×ΔVcs), [Eq-5]
where Gm is a transconductive coefficient of the voltage-to-current converter 7002. In addition, the capacitor CX has the capacitance
CX=Vinmax[(Tmin_max+Tp)Tp×3Rcs×Gm]/(V2×Lm). [Eq-6].
By simulating the minimum on-time modulation circuit 70, the relationship between the minimum on-time Tmin and the input voltage Vin is also obtained, as shown by curve 94 in
While the present invention has been described in conjunction with preferred embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and scope thereof as set forth in the appended claims.
Claims
1. A control circuit for a flyback power converter including a transformer and a power switch connected to a primary side of the transformer, to switch the power switch so as for the transformer to convert an input voltage into an output voltage, the control circuit comprising:
- a sampling and holding circuit extracting a variation of a sensing signal during a preset time period, wherein the sensing signal is a function of the input voltage;
- a compensation circuit connected to the sampling and holding circuit, compensating a first feedback signal according to the variation of the sensing signal to generate a second feedback signal varying with the input voltage, wherein the first feedback signal is a function of the output voltage; and
- a pulse width modulation circuit connected to the compensation circuit, determining an on-time of the power switch according to the sensing signal and the second feedback signal.
2. The control circuit of claim 1, wherein the sampling and holding circuit comprises:
- a first switch;
- a capacitor connected to the first switch;
- a second switch shunt to the capacitor, being switched by a first clock to reset the capacitor; and
- a constant on-time generator connected to the first switch, generating a second clock having a constant on-time equal to the preset time period according to the first clock, to switch the first switch for the sensing signal to charge the capacitor to obtain the variation of the sensing signal.
3. The control circuit of claim 1, further comprising a gain stage connected to the sampling and holding circuit, generating a gain signal proportional to the variation of the sensing signal.
4. The control circuit of claim 3, wherein the compensation circuit comprises an adder connected to the gain stage, subtracting the gain signal from the first feedback signal to generate the second feedback signal.
5. The control circuit of claim 1, wherein the pulse width modulation circuit comprises:
- a comparator connected to the compensation circuit, comparing the sensing signal with the second feedback signal to generate a comparison signal; and
- a flip-flop having a setting input to receive a clock, a resetting input to receive the comparison signal, and an output to provide a control signal to switch the power switch.
6. The control circuit of claim 5, further comprising a hysteresis comparator generating a mask signal according to the first feedback signal and a preset voltage to mask the clock.
7. The control circuit of claim 5, further comprising a transconductive amplifier to modulate a frequency of the clock according to the first feedback signal and a preset voltage.
8. The control circuit of claim 1, further comprising a minimum on-time modulation circuit connected to the sampling and holding circuit, to modulate a minimum on-time of the power switch according to the variation of the sensing signal.
9. The control circuit of claim 8, wherein the minimum on-time modulation circuit comprises:
- a capacitor;
- a current source connected to the capacitor and the sampling and holding circuit, providing a current varying with the variation of the sensing signal to charge the capacitor to generate a first signal;
- an error amplifier connected to the sampling and holding circuit, generating a second signal according to the variation of the sensing signal and a preset voltage; and
- a comparator connected to the capacitor and error amplifier, determining the minimum on-time of the power switch according to the first and second signals.
10. A control circuit for a flyback power converter including a transformer and a power switch connected to a primary side of the transformer, to switch the power switch so as for the transformer to convert an input voltage into an output voltage, the control circuit comprising:
- a sampling and holding circuit extracting a variation of a sensing signal during a preset time period, wherein the sensing signal is a function of the input voltage; and
- a minimum on-time modulation circuit connected to the sampling and holding circuit, modulating a minimum on-time of the power switch according to the variation of the sensing signal.
11. The control circuit of claim 10, wherein the minimum on-time modulation circuit comprises:
- a capacitor;
- a current source connected to the capacitor and the sampling and holding circuit, providing a current varying with the variation of the sensing signal to charge the capacitor to generate a first signal;
- an error amplifier connected to the sampling and holding circuit, generating a second signal according to the variation of the sensing signal and a preset voltage; and
- a comparator connected to the capacitor and error amplifier, determining the minimum on-time of the power switch according to the first and second signals.
12. A control method for a flyback power converter including a transformer and a power switch connected to a primary side of the transformer to be switched so as for the transformer to convert an input voltage into an output voltage, the control method comprising:
- providing a sensing signal which is a function of the input voltage;
- providing a first feedback signal which is a function of the output voltage;
- sampling and holding the sensing signal to extract a variation of the sensing signal during a preset time period;
- generating a second feedback signal varying with the input voltage by compensating the first feedback signal with the variation of the sensing signal; and
- determining an on-time of the power switch according to the sensing signal and the second feedback signal.
13. The control method of claim 12, wherein the step of sampling and holding the sensing signal comprises:
- providing a first clock;
- according to the first clock, generating a second clock having a constant on-time;
- using the second clock to switch a first switch serially connected to a capacitor for the sensing signal to charge the capacitor to obtain the variation of the sensing signal; and
- using the first clock to switch a second switch shunt to the capacitor to reset the capacitor.
14. The control method of claim 12, wherein the step of generating a second feedback signal varying with the input voltage comprises:
- generating a gain signal proportional to the variation of the sensing signal; and
- subtracting the gain signal from the first feedback signal to generate the second feedback signal.
15. The control method of claim 12, wherein the step of determining an on-time of the power switch comprises:
- triggering the on-time of the power switch in response to a clock; and
- terminating the on-time of the power switch when the sensing signal reaches the second feedback signal.
16. The control method of claim 15, further comprising generating a mask signal according to the first feedback signal and a preset voltage to mask the clock.
17. The control method of claim 15, further comprising generating a modulation signal according to the first feedback signal and a preset voltage to modulate a frequency of the clock.
18. The control method of claim 12, further comprising modulating a minimum on-time of the power switch according to the variation of the sensing signal.
19. The control method of claim 18, wherein the step of modulating a minimum on-time of the power switch comprises:
- providing a current varying with the variation of the sensing signal to charge a capacitor to generate a first signal;
- generating a second signal according to a difference between the variation of the sensing signal and a preset voltage; and
- determining the minimum on-time of the power switch according to the first and second signals.
20. A control method for a flyback power converter including a transformer and a power switch connected to a primary side of the transformer to be switched so as for the transformer to convert an input voltage into an output voltage, the control method comprising:
- providing a sensing signal which is a function of the input voltage;
- sampling and holding the sensing signal to extract a variation of the sensing signal during a preset time period; and
- modulating a minimum on-time of the power switch according to the variation of the sensing signal.
21. The control method of claim 20, wherein the step of modulating a minimum on-time of the power switch comprises:
- providing a current varying with the variation of the sensing signal to charge a capacitor to generate a first signal;
- generating a second signal according to a difference between the variation of the sensing signal and a preset voltage; and
- determining the minimum on-time of the power switch according to the first and second signals.
Type: Application
Filed: Jul 9, 2009
Publication Date: Jan 14, 2010
Patent Grant number: 8472214
Applicant: RICHTEK TECHNOLOGY CORP. (HSINCHU)
Inventors: PEI-LUN HUANG (Zhubei City), YU-MING CHEN (HSINCHU CITY), TZU-CHEN LIN (TAIPEI CITY)
Application Number: 12/500,019
International Classification: H02M 3/335 (20060101);